This invention relates to aluminum alloy sheet products and methods of making them. In an important specific aspect, to which detailed reference will be made herein for purposes of illustration, the invention is directed to aluminum alloy sheet products having strength, formability, corrosion resistance, and age-hardening properites advantageously suitable for forming automobile body members.
It has long been conventional to fabricate automobile body members (i.e. hoods, trunk lids, doors, floor panels, and the like) of sheet steel, but increasingly in recent years, aluminum alloy sheet has been used for such members, because of the relatively light weight of aluminum as well as for other reasons. In order to be acceptable for automobile body sheet, however, an aluminum alloy must not only possess requisite characteristics of strength and corrosion resistance, but must also exhibit good formabability. Since aluminum alloy body sheet must often be formed in dies designed for mild steel, the relatively low formability of some aluminum alloys as compared to steel, particularly in respect of bendability, has limited use of aluminum alloys as a substitute for steel in automotive body members. Moreover, the hem flanging operation frequently employed to join the exterior panel member to the inner support panel member of a commercial automotive body panel requires that the metal be subjected to a 180.degree. bend over a single sheet thickness; some of the stronger aluminum alloys are incapable of producing a crack-free hem flange, necessitating resort to a so-called rope hem flange which incorporates a larger bend.
As a further consideration bearing on suitability of aluminum alloys for automotive body use, the advent of stretch-draw forming technology on a commercial scale for large body panels has shifted the emphasis in sheet properties from avoidance of necking during drawing to promotion of uniform stretching and minimization of springback. The objective of stretch-draw techniques is to promote uniform deformation such that all the blanked sheet metal receives at least some straining and no localized regions are subjected to excessively large strains; hence, for these operations, alloy sheet should have a high work-hardening capability at relatively low strains, as well as a low springback tendency.
An increasingly important consideration in the selection of a suitable aluminum alloy for an automotive "stamping" is its versatility. For with an alloy that possesses the proper combination of strength and formability, a wide range of components may be manufactured. A case in point is that of an automotive hood, which consists of a difficult-to-form inner support panel and a strong, dent-resistant outer skin. Historically, this has required the selection of two alloys: for example, the more formable AA-6009 alloy for the inner panel, and the stronger 6010 alloy for the outer panel. This necessity of purchasing two alloys for such a component is an undesirable feature, resulting in increased stock and scrap recycling problems. Hence a single alloy, suitable for both the inner and outer panels is a very desirable feature.
In addition, since formed automotive body panels are customarily painted and then subjected to a paint-baking treatment typically at a temperature of 177.degree.-204.degree. C., the sheet alloy used for such panels should not be adversely affected by this thermal treatment. Preferably, the alloy should exhibit an age-hardening response to the paint-bake treatment, i.e. it should increase in strength when subjected thereto.
Aluminum alloys previously proposed for use in automotive body members have included the alloys bearing the Aluminum Association designations AA 2036 and AA X2038. A disadvantage of the 2036 alloy is that it does not age harden during typical paint-bake cycles, but on the contrary may even undergo reduction in strength during paint-baking. AA X2038 alloy does exhibit appreciable age hardening at the usual paint-bake temperatures, but both of these alloys, which are high-copper alloys, have relatively low corrosion resistance.
It has also been proposed, for example in U.S. Pat. No. 4,082,578, to use Al-Mg-Si alloys for making automotive body panels. The patent refers to alloys having a content (in percent by weight) of 0.4-1.2% Si, 0.4-1.1% Mg, 0.2-0.8% Mn, 0.05-0.35% Fe, and 0.1-0.6% Cu, balance aluminum and incidental elements and impurities. Two examples of specific alloys of this type are those identified by the Aluminum Association's designations AA 6009 and AA 6010. Current published composition ranges and limits for the aforementioned alloys, and for two other known alloys of the Al-Mg-Si type (AA 6011 and AA 6110), used respectively for venetian blind slats and for wire and nails, are set forth in Table I.
TABLE I __________________________________________________________________________ Range or Maximum (Percent by Weight).sup.(1) AA 2036 AA X2038.sup.(2) AA 6009 AA 6010 AA 6011.sup.(3) AA 6110 __________________________________________________________________________ Cu 2.2-3.0 0.8-1.8 0.15-0.6 0.15-0.6 0.40-0.9 0.20-0.7 Mg 0.30-0.6 0.40-1.0 0.40-0.8 0.6-1.0 0.6-1.2 0.50-1.1 Si 0.50 0.50-1.3 0.6-1.0 0.8-1.2 0.6-1.2 0.7-1.5 Fe 0.50 0.6 0.50 0.50 1.0 0.8 Mn 0.10-0.40 0.10-0.40 0.20-0.8 0.20-0.8 0.8 0.20-0.7 Ti 0.15 0.15 0.10 0.10 0.20 0.15 Cr 0.10 0.20 0.10 0.10 0.30 0.04-0.25 Zn 0.25 0.50 0.25 0.25 1.5 0.30 __________________________________________________________________________ .sup.(1) In each of the specified alloys, the maximum content of other elements is 0.05% each, 0.15% total; and the remainder of the alloy is aluminum. .sup.(2) Ga, up to 0.05% max; V, up to 0.05% max. .sup.(3) Ni, up to 0.20% max.
Alloys AA 6009 and AA 6010 both undergo age-hardening during paint baking and have substantially higher resistance to corrosion than alloys AA 2036 and AA X2038. AA 6009, which has a nominal content of 0.50% Mg and 0.80% Si, exhibits lower strength but higher formability than AA 6010, which has a nominal content of 0.80% Mg and 1.00% Si. Especially for automotive body use, it would be desirable to provide an aluminum alloy sheet having higher strength than AA 6009 and greater formability than AA 6010.